Oil shale is a resource associated with coal. The refining of raw oil and its byproducts using oil shale dry distillation technology had supported the economic development of China in the years of oil shortage. However, the amount of research and development of this technology had shrunk year by year. The shale oil resource contained in the oil shale around the world amounts to about 366.2 billion tons, at least 50% more than the traditional petroleum resource. The prospected and predicted amount of oil shale in China is 483.2 billion tons, which contains 29 billion tons of shale oil resource, ranking the fourth place in the world.
The countries that use oil shale to produce shale oil include Estonia, Brazil, China and Australia, etc. Brazil produced 195 kilotons of shale oil in 1999, China produced 200 kilotons of shale oil in 2006, and Australia produced 28 kilotons of shale oil in 2001, rising to 60 kilotons in 2002. Along with the advancement of technological development and application, as well as the strengthened awareness of environmental protection, it is a global trend to make full use of oil shale resource. It has become a trend to utilize oil shale in a more combined or diversified way.
At the beginning of 1990s, Fushun Mining Group Co., Ltd. adopted the concept of circular economy, and restarted the complex use of and research on oil shale, wherein emphasis is put on the refining of oil from oil shale.
At present, in the production of oil shale dry distillation, the output dry distillation product containing shale oil vapor, water vapor and dry distillation gas entrains a lot of shale powder dusts which form an oleophilic matter after adsorbing extremely fine oil fog during dust scrubbing, wherein the black, semi-solid clay-like matter that is produced during rigorous agitation and contains oil, water, and powder dust is called “oil sludge”. The output of oil sludge varies with different operating conditions in production, generally accounting for 3-5‰ of the output of raw shale oil. The annual output of oil sludge in a shale refining plant is about 450-750 tons (based on 15 t/a raw oil output). The oil sludge in an oil shale refining plant is composed of a certain mass percentage (10-40%) of raw oil and 40-65% by mass of shale powder dust, heavy metal ions, inorganic salts and like substances, and has a density of 1.5˜1.8 g/cm3.
Oil sludge of shale dry distillation features high oil content and significant emulsification of oil phase, water phase and solid phase. Due to fine granularity and high density, shale powder dust is hard to separate by means of conventional methods, and may result in environmental pollution if not handled properly. Over the years, the shale oil industry has spared no effort and has invested tremendous manpower and resources, and has initiated scientific research projects. However, no effective method has been obtained. Some treatment methods adopted in current practice have limited effectiveness and can alleviate environmental pollution to some degree. However, these methods are still a far cry, in terms of quality and efficiency, from meeting the demands of the industrial development of shale oil refining and storage. Hazard-free green treatment of oil containing solid waste for elimination of environmental risk has good social and environmental benefit. In contrast, scientific green treatment of oil sludge not only can reclaim raw shale oil effectively to transform waste into treasure, but also provide good economic benefit.
Dry distillation of oil shale produces shale oil and oil sludge, together with large amount of dry distillation waste water. The product is essentially comprised of the following parts:
(1) associated water in the mixed gas emitted from the top of a dry distillation furnace, which is composed of the water contained in raw oil shale, the water contained in the main blowing air, the water evaporated from the water basin and a small amount of combined water, wherein this part of associated water passes through the whole process of the dry distillation of oil shale, exists in the form of water vapor, gradually condenses into water after entering the gathering pipe of the mixed gas, and is the main source of dry distillation waste water;
(2) drained water from processing and scrubbing of dry distillation product, which is the water generated during processing and scrubbing of dry distillate and recovery of shale oil;
(3) entrained water in the mixed gas emitted from the gathering pipe, which is generated from a portion of the oil/water drops entrained in the mixed gas emitted from the gathering pipe section and the incompletely cooled tar gas that continues cooling in a gas tower; and
(4) drained water from circulatory cooling, which is the water generated during cooling the circulatory gas.
Dry distillation waste water has various types of pollutants, with complex compositions at high concentrations, which contains a relatively large amount of environmental pollutants such as organic sulfur, organic nitrogen, naphthenic acid compounds and fused-ring aromatic hydrocarbon compounds, etc., in addition to petroleum like substances and volatile phenol compounds conventionally found in natural oil plant. It is an organic waste water that is widely recognized as being difficult to handle, and is one of the industrial waste water types for which treatment technology is imported as of national importance. Waste water from dry distillation of shale has been treated in domestic and overseas laboratories successfully, but the experimentation came to a standstill due to relatively high facility investment and non-ideal economic benefit. Waste water treatment is relatively difficult.
In the common production process of oil shale dry distillation at present, the coke oven gas and tar from the outlet of the dry distillation furnace enter the gathering pipe at ambient pressure, wherein the oil sludge (a mixture of heavy oil and powder dust) is removed from the gas by circulatory water scrubbing. The scrubbing process is also a process for cooling and separation of the coke oven gas and coal tar.
Currently, the method for treating the dry distillation waste water from the gathering pipe section in the Fushun type oil shale dry distillation process is simple settlement of the waste water containing oil sludge in a sludge collecting tank, wherein the treated sludge is recycled to the gathering pipe directly. Specifically, the gathering pipe is inclined such that, after scrubbing, the scrubbing water, oil sludge and shale oil flow along the gathering pipe into the sludge collecting tank to achieve precipitation and separation. The water in the sludge collecting tank is drawn into the gathering pipe using a sludge collecting tank pump for circulatory use, and the oil sludge is discharged periodically from the sludge collecting tank and sent to the construction sector for further use. According to the method for separating oil sludge, sludge is generally removed by direct precipitation in the sludge collecting tank and multi-stage scrubbing. Apparently, this is quite undesirable for the collection of oil sludge.
Although zero discharge of waste water can be realized and a lot of costs for treating waste water can be saved by doing so, concern exists in heavy corrosion of equipment and in clogging and corrosion of pipes.
In the newest oil shale process, a scrubbing/saturation tower includes a gas tower and an air tower, and the scrubbing section is positioned at the bottom of these two towers. The gas tower acts to scrub or cool the forward mixed gas and circulatory gas, and the air tower acts to scrub or cool the forward mixed gas and main blowing air. The problem that the main blowing air entrains sludge is avoided in the air tower because all of the oil sludge has been removed in the preceding gas tower and the scrubbing section is located below the gas tower. The lower half of the gas tower is used to carry out secondary cooling and scrubbing of the mixed gas, while the upper half thereof carries out a new stage of scrubbing of the circulatory gas recycled from the cooling tower, to remove the tar therein thoroughly. A water seal is positioned at the interface between the upper and lower parts to prevent intermingling of the circulatory gas and the mixed gas. When the circulatory water is sprinkled down from the top of the tower, it passes through the water seal by virtue of overflowing, and scrubs or cools the circulatory gas and the mixed gas in sequence. By doing so, various problems of the scrubbing/saturation tower have been solved. Addition of another stage of scrubbing or cooling process flow to each of the mixed gas and the circulatory gas further promotes the recovery yield of shale oil and the cooling effect, and prevents the problem of scale formation after oil dirt enters the heating furnace.
After scrubbing or cooling the circulatory gas and the mixed gas, the scrubbing/cooling water contains a relatively large amount of oil sludge and shale oil. Addition of a gas tower sludge remover and a gas tower oil remover at the bottom of the gas tower may well meet the demand of the circulatory use of the scrubbing/cooling water, and facilitate the recovery of shale oil and oil sludge as well.
So far, the treatment of oil sludge and waste water according to the existing oil shale dry distillation technology has never achieved fine separation or solved the environmental pollution problem associated with emission. Therefore, an urgent need exists in the art for development of a new treatment technology suitable for long-term operation to solve the above problems.